ECG Blog #308: Funny P Waves & Acute Inf. STEMI

The ECG in Figure-1 was obtained from a 70-year old man with longstanding hypertension. The patient was in for his yearly check-up. He denied symptoms.

• A preliminary diagnosis of an acute inferior STEMI was made on seeing the ECG in Figure-1. Do you agree?

Figure-1: The initial ECG in today's case.

MY Thoughts on the ECG in Figure-1:

Beginning with Rate & Rhythm — the long lead II rhythm strip shows the rhythm to be regular at ~80-85/minute. The QRS complex is narrow. A P wave is seen before each QRS complex  — but this P wave is negative in lead II. Therefore the rhythm is not sinus. Instead — this is either a low atrial rhythm — or — an accelerated junctional rhythm.

• NOTE: Despite short duration of the PR interval — this does not distinguish been a low atrial vs junctional rhythm because it is speed of conduction (rather than distance from the SA node) that determines PR interval duration.

Continuing with Interpretation of ECG #1:

• Intervals: The QRS and QTc intervals are normal.
• Chamber Enlargement: None.
• Q-R-S-T Changes: There is an isolated but large Q wave in lead III. R wave progression is normal — with transition (where the R wave becomes taller than the S wave is deep) occurring normally between leads V3-to-V4. The principal finding of concern is what appears to be ST elevation in each of the inferior leads — with what appears to be reciprocal ST depression in lead aVL. The rest of the ECG is unremarkable.

QUESTION:

• Is the ST elevation in Figure-1 a "real" finding?

ANSWER: The Emery Phenomenon

The appearance of ST elevation in the inferior leads of Figure-1 reflects the Emery Phenomenon — in which the oppositely-directed atrial repolarization wave (ie, the T of the P wave) produces a "pseudo"-ST elevation effect because of the relatively large size of the negative inferior lead P waves, with short PR interval.

• Most of the time — the Tp (also known as the "Ta" or atrial T wave) is hidden within the QRS complex. But on those uncommon occasions when a large negative P wave with short PR interval is seen in the inferior leads — the resultant oppositely-directed Tp may simulate acute inferior infarction (See My Comment in the June 3, 2020 post in Dr. Steve Smith's ECG Blog for discussion of the Emery Phenomenon in the context of a case that went to cath because of this "pseudo"-ST elevation).

To illustrate this phenomenon — I’ve adapted Figure-2, which I’ve taken from a 2015 post on the ECG Rhythms website.

• As suggested in Figure-2 — the atrial repolarization wave (ie, the T of the P wave) is always present — but with sinus rhythm, the timing of the Tp will largely coincide with the timing of the QRS complex, and therefore not be noticed on the ECG (dotted RED half circle, seen to the left in Figure-2).
• As shown in Figure-2 — the Tp will be oppositely directed to the P wave. Therefore, with normal sinus rhythm (in which by definition, the P wave will be upright in lead II) — the TP will be negative.
• IF the P wave in lead II is negative (as may occur with either a low atrial or junctional rhythm) — then the Tp will be upright (dotted RED half circle, seen to the right in Figure-1). If the Tp wave is large in size and upright — it may distort the end of the QRS complex, and produce the false impression of ST elevation.

Figure-2: Illustration of the Emery Phenomenon. (I have adapted this Figure from the 2015 post by Dr. Bojana Uzelac on Armel Carmona’s ECG Rhythms website).

KEY Points:

• The size of the Tp wave will be proportional to the size of its P wave. A small P wave will produce a correspondingly small Tp wave. A large P wave will produce a much bigger Tp wave.
• Actually, the effect of the oppositely-directed atrial repolarization wave ( = the Tp — also known as the "Ta" or atrial T wave) will be even larger than shown above in Figure-2 — because normal duration of the Ta wave is significantly longer (up to 2-3 times longer) than normal P wave duration (Francis). This may account for an exaggerated effect on the ST segment when the P wave is large.
• That said — I preserved the same relative proportions in Figure-2 as were seen in the original version of this Figure taken from the ECG Rhythms website. Note that the PR interval for the negative P wave in Figure-2 is almost as long as the PR interval for normal sinus rhythm. But IF the PR interval for the negative P wave in lead II is much shorter (as occurs in today’s case) — then the upright Tp wave that will be seen with a low atrial rhythm will be further displaced to the right, which will produce a much greater degree of pseudo-ST-elevation!

The CASE Continues:

10 minutes later in today's case — a repeat ECG was done (Figure-3).

• What has happened in Figure-3?

Figure-3: Comparison between the initial ECG — and the repeat ECG done 10 minutes later.

ANSWER:

The repeat ECG (bottom tracing in Figure-3) — shows return of normal sinus rhythm with an upright P wave in lead II, and an overall slower heart rate than was seen in ECG #1.

• Following this return of the normal upright sinus P wave in lead II (as well as in the other inferior leads) — there is no longer any ST elevation in the inferior leads of ECG #2. In addition — the small amount of J-point ST depression that had been seen in lead aVL of ECG #1 is no longer present.
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• Since ECG #2 was obtained just 10 minutes after ECG #1, without any change in the patient's clinical condition — this resolution of inferior lead ST elevation (and of the J-point ST depression in lead aVL) — confirms that the ST elevation that had been seen in ECG #1 was not real. Instead — it was simply an effect of the Emery Phenomenon, brought about as a result of the large-amplitude negative inferior lead P waves with short PR interval that were seen in ECG #1.

KEY Point:

• Although there is no longer any ST elevation in ECG #2 — the T waves in the inferior leads still look "hypervoluminous" (ie, each of the inferior lead T waves either equal or exceed amplitude of the R wave in the same lead — and each of these T waves have a broader-than-expected base).
• In addition — despite a QRS complex in lead aVL that is not predominantly negative — the T wave in this lead is still inverted.
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• PEARL: The importance of the History can not be overstated. IF I was shown ECG #2 and told that the patient with this ECG was complaining of new-onset chest pain — I would interpret this tracing as showing hyperacute T waves in each of the inferior leads, with reciprocal change in lead aVL. My diagnosis would be acute RCA (Right Coronary Artery) occlusion until proven otherwise.
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• However, the 70-year old man in today's case was completely asymptomatic — with the reason for getting an ECG being "routine", as part of this patient's regular check-up. In view of this information — it is highly likely that nothing acute is going on in ECG #2.
• I suspect that review of this patient's medical record, looking for a previous ECG for comparison would quickly resolve all questions by showing a longstanding similar ST-T wave appearance.

Take-Home MESSAGE:

Be aware of the Emery Phenomenon! All providers whose work entails ECG interpretation will occasionally encounter patients with a low atrial or junctional rhythm (with large negative P waves and a short PR interval in the inferior leads) — that produces inferior lead ST elevation (often with reciprocal change in lead aVL) that simply is not real.

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Acknowledgment: My appreciation to Kianseng Ng (from Kluang, Malaysia) for the case and this tracing.

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Additional Relevant Material to Today's Case:

• See ECG Blog #185 — for review of the Systematic Ps, Qs, 3R Approach to rhythm interpretation.
• See ECG Blog #205 — Reviews my Systematic Approach to 12-lead ECG Interpretation.
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• For more on distinction between Low Atrial vs Junctional Rhythm — Please see My Comment at the BOTTOM of the page in the January 28, 2019 post in Dr. Smith's ECG Blog.
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• See ECG Blog #290 — for another example of the Emery Phenomenon.
• And for another case of the Emery Phenomenon — Please see My Comment at the BOTTOM of the page in the June 3, 2020 post in Dr. Smith's ECG Blog.